Two years ago Dr Hamer had shown in a study of the family histories of 114 homosexual men that being gay tends to be inherited. He found that 13.5 per cent of the brothers of these men were also gay, compared with just 2 per cent in the general population, a significant difference that appeared to show an inherited component to sexual orientation.
Dr Hamer also found that maternal uncles and maternal male cousins were more likely than by chance alone to be homosexual, indicating that whatever the inherited component was it was likely to be transmitted through the maternal line. This implicated the X chromosome which men inherit from their mothers (they receive their Y chromosome from their fathers).
He conducted detailed molecular studies of the genetic material - the DNA blueprint - of the X chromosomes of 40 pairs of gay brothers to study ''genetic markers'', which are rather like signposts on a road map. He found 33 of the paired brothers had co-inherited genetic markers on the same region of the X chromosome, known as Xq28.
This region represents just 0.02 per cent of the entire genetic make- up of humans, and yet it could contain up to several hundred genes. Although Dr Hamer had not found a ''gay gene'', he had discovered convincing evidence that this region influences sexual orientation in some men.
When this work emerged in a blaze of publicity, there was an outcry from people who for one reason or another had vested interests in showing homosexuality could or could not be inherited.
Paradoxically, Dr Hamer does not himself believe in a gay gene despite trying more than any other scientist to prove the existence of a genetic - and therefore inherited - component to sexual orientation.
The primary conclusion of his latest work, published yesterday in the journal Nature Genetics, is that there is a region on the X chromosome that influences variations in sexual orientation in men, but not in women. Though Dr Hamer adds: ''That does not mean we have found a gay gene.''
Despite his protestations, however, this has not stopped the world from interpreting his research in this simplistic way. Indeed, he has admitted in the past that finding a gene for homosexuality may be possible. And it is likely that someone, somewhere will attempt to use his work to develop a test for gayness, something he has said he would do everything in his power to stop.
The significance of Dr Hamer's latest work is that it answers some of his severest critics, who have called into question the validity of his earlier research, published in Science in July 1993. Dr Hamer, who refuses to answer questions on allegations that he is being investigated by the US government's Office of Research Integrity, robustly defends all his research. ''This current work is not under any question whatsoever'' and the ''main findings'' of the previous study ''have never been challenged'', he said. Any further questions on that topic he refers to his lawyers.
The deep controversy stirred up by Dr Hamer's work is a rare phenomenon in the arcane world of genetics research, which usually concerns itself with identifying and eradicating or at least alleviating inherited diseases. Homosexuality, however, is not a disease nor an abnormality.
Some disagree. When Dr Hamer's findings first appeared, a number of commentators, including at least one religious leader, appeared to countenance the prospect of a prenatal test for a gay gene, with the possibility of abortion. (The Daily Mail heralded the research with the famous headline: ''Abortion hope after 'gay genes' findings''.)
At another extreme, the research was condemned by some gay groups and scientists who felt it proved nothing except to demonstrate the gullibility of those who want to believe in the genetic determinism of complex human behaviours. ''If you are investigating the causes of homosexuality,'' said gay activist Peter Tatchell, ''why not look at the causes of heterosexuality? The implication is that heterosexuality is normal, natural and unproblematic.''
Curiously, in the US at least, a number of gay groups welcomed the research, arguing that it demonstrated the natural condition of homosexuality, akin to inheriting blue eyes or brown hair, instead of regarding it as the consequence of perverse choice. Just as legislation can be enacted to stop discrimination on grounds of race or sex (both genetically inherited) then the same argument, they believed, could apply to homosexuality.
The overriding concern, however, has been that Dr Hamer's work would lead to the development of a genetic test. Last year, Dr Hamer addressed this fear at the annual meeting of the American Association for the Advancement of Science. ''People are worried that eventually someone - scientists, the military or insurance companies - might try to develop blood tests for sexual orientation, or a prenatal test so that expectant mothers could abort a foetus at risk of being gay. I think this would be wrong, unethical, and a terrible abuse of research. It's wrong to discriminate on the basis of genes.''
Dr Hamer said he would try to prevent anyone from developing such a test. ''We'll have the intellectual property rights for that work and that means we will be able to not give those rights to people who are likely to commercialise them.''
This left some people asking what then was the point of doing such research? Dr Hamer's reply was that it furthered our understanding of an important aspect of human nature - sexual orientation. ''The Aids epidemic has taught us, too bitterly, that we have more to fear from ignorance than from new knowledge about human sexuality.''
Since 1993, Dr Hamer has expanded his research to include an extensive collaboration with scientists from Colorado University and the prestigious Whitehead Institute in Cambridge, Massachusetts. Their latest findings, on another 33 pairs of gay brothers and 11 pairs of heterosexual brothers, confirmed the earlier work.
They also found no evidence that the critical Xq28 region of the X chromosome had anything to do with sexual orientation in lesbian women.
According to Dr Hamer, this means he is twice as sure as before that this part of the X chromosome bears some responsibility for the inherited component of being gay - and, as he was already 99 per cent sure before this work was finished, it means he is now pretty confident he has got it right.
Such confidence is likely to provoke still further those who criticise the meaningfulness of such research. And Dr Hamer will continue to generate widespread controversy, especially if the day comes when he or someone else is able to announce the discovery of a true ''gay gene'', which will mean a medical test for homosexuality will not be far behind.
Arguments for and against
The idea that [genetics] might be the reason for homosexuality does not take into consideration bisexuality. It polarises homosexuality and heterosexuality rather than seeing them as part of a continuum.
David Fernbach, homosexual activist, July 1993
Some groups on the right, the religious fundamentalists, will say, "Here we have the gene for homosexuality, let's get rid of it." These are the same people who in the Thirties in Germany looked for a master race.
Michael Cashman of Stonewall, the gay rights group, July 1993
If you are investigating the causes of homosexuality, why not look at the causes of heterosexuality?
Peter Tatchell, July 1993
Just because something is ethically complicated doesn't mean it shouldn't be investigated.
Dr Bryan Sykes, a geneticist at the Institute of Molecular Medicine in Oxford, July 1993
We are carried along on a pell-mell rush towards biological determinism, a tidal wave of enthusiasm for all things genetic.
Professor Steven Rose of the Open University, July 1993
We have not found the gene, which we don't think exists, for sexual orientation.
Dr Dean Hamer, geneticist at the US National Cancer Institute, July 1993
You can never say we're 100 per cent sure, but we're twice as sure as before. We are better than 99 per cent sure that it's not a fluke.
Dr Dean Hamer, October 1995
Intellectually, what do we gain by finding out there's a homosexual gene? Nothing, except an attempt to identify those people who have it and then open them up to all sorts of experimentation to change them.
Darrell Yates Rist, co-founder of the
Gay and Lesbian Alliance Against Defamation in the United States, July 1993
In my view, the scientific evidence presently available points to a strong influence of nature and only a modest influence of nurture.
Professor Simon LeVay, US neuroscientist and author of 'The Sexual Brain', July 1993
The gene may do the same thing in men and women, so if you're a man you'll be gay, but in a woman she'll be attracted to men - perhaps more so than usual - and she'll have enough children to make up for those that gay men won't have.
Dr Dean Hamer, October 1994
Homosexuality is a disability and if people wish to have it eliminated before they have children - because they wish to have grandchildren or for other reasons - I do not see any moral objection for using genetic engineering to limit this particular trend. It would be like correcting many other conditions such as infertility or multiple sclerosis.
Lord Jakobovits, former chief rabbi, July 1993
Abortion hope after "gay genes" findings
'Daily Mail', headline, July 1993
HOW GENES SHAPE THE MIND
The Amish people of Pennsylvania are ideal for the research of human genetics because they seldom marry outside their community. Studies suggesting that the Amish's high incidence of manic depression could be traced to DNA were later discredited.
In 1990, US researchers announced a link between alcoholism and genes affecting the way chemical signals were transmitted in the brain's "pleasure centre". Others failed to confirm this, and still others suggest it is unlikely that, say, Russia's problems with drunkenness can be linked to the genetic make-up of the population.
Thomas Bouchard of the University of Minnesota has conducted the most extensive study comparing identical twins (who share all their genes) raised in the same family with those rare cases where they had been raised apart (and therefore have not shared the same environment). He claimed that a tendency to religious belief is in the genes. Other scientists are sceptical about the feasibility of separating genetic from environmental influences on human behaviour.
"Genes for intelligence" have an unhappy history of racism and scientific fraud, but there is clearly a genetic influence on IQ as some inherited disorders can cause mental retardation. Many researchers doubt that the genetics of severe mental retardation will throw any light on the variation in normal intelligence. However, some scientists are trying to find a genetic basis to ordinary variation in intelligence.
In 1988, researchers in London claimed they had found a genetic marker in the DNA of schizophrenics. They issued a press release asserting that the gene causing schizophrenia had been found. It hadn't, and the finding was quietly withdrawn several years later. However, the search for genetic abnormalities associated with schizophrenia continues in both the US and Europe.
In 1993, Dutch researchers found a family suffering a syndrome of borderline mental retardation and behaviour including "arson, attempted rape and exhibitionism". They had a rare genetic mutation affecting a chemical in the brain. American talk-show radio hosts suggested sterilising those who carry "the mean gene".
Last year, US scientists discovered clear evidence that reading disability can be hereditary during a study of nine families with a history of dyslexia, each spanning three generations.
Text: Tom Wilkie